Structural basis for substrate specificity of single domain APOBEC3s Members of the APOBEC3 (A3) family of cytidine deaminases provide a first line of defense against HIV; however, HIV evades A3 restriction factors through viral infectivity factor (Vif), which antagonizes A3. The overall objective of this proposal is to elucidte the structural basis for substrate specificity of single domain APOBEC3s toward understanding HIV restrictions. A3s can inhibit HIV replication by introducing lethal mutations in the viral genome during reverse transcription. Differences in substrate specificities for deaminating ssDNA among A3 enzymes vary, yet the structural basis for substrate sequence specificity is not well understood. I hypothesize that topological and chemical differences in the active site of A3 enzymes are responsible for their substrate sequence specificities. I propose the following three aims to test my hypothesis: elucidate [1] the substrate sequence dependence of A3 affinity to ssDNA, [2] the mechanism of A3 interaction with substrate ssDNA, and [3] the correlation between homology and specificity of the active site of active A3 Z domains. Results of this proposal will not only contribute to a more comprehensive understanding of the structural basis for A3 function, but will also provide an essential foundation for new innovative anti-HIV drug development strategies.